C->W: GET /concert HTTP/1.1
      Host: www.content.com

W->C: HTTP/1.1 200 OK
      Content-Type: application/sdf

      (session
        (id rtsp://live.content.com/concert)
      )

C->M: GET rtsp://live.content.com/concert RTSP/1.0 1

M->C: RTSP/1.0 200 1 OK
      Content-Type: application/sdf
      (session (all
        (media (t audio) (id music) (a IP4 224.2.0.1) (p 3456))
      ))

C->M: PLAY rtsp://live.content.com/concert/music RTSP/1.0 2

M->C: SESSION concert RTSP/1.0
      Content-Type: application/sdf
      (session (all
        (media (t audio) (id music))
        (media (t text) (id lyrics))
      ))

C->M: PLAY rtsp://live.content.com/concert/lyrics RTSP/1.0